We have discovered two metabolically distinct receptors on the liver membranes of dogs, the apo-B,E and the apo-E receptor. The apo-B,E receptor is analogous to the LDL receptor of fibroblasts and binds lipoproteins containing the B and/or E apoproteins. The apo-E receptor binds lipoproteins containing the E apoprotein but not normal LDL and is postulated to be the receptor which clears chylomicron remnants. We propose to isolate and characterize these two receptors and to determine their physiological function. The receptors will be solubilized and partially purified to verify the existence of each receptor as distinct proteins. The purification will then proceed using techniques such as ion-exchange and Sepharose-apo-E HDLc affinity chromatography until the E receptor is isolated as a pure protein. In addition, the molecular weight of the receptors will be determined by radiation inactivation. To determine the function of these receptors, we will use two complimentary methods. In the first approach we will use a combination of in vivo lipoprotein turnover to measure lipoprotein catabolism coupled with an in vitro binding assay to measure the levels of hepatic lipoprotein receptors. These studies will be conducted on dogs that have been metabolically perturbed by lipid altering drugs, hormones or diet in order to explore the role of hepatic receptors in lipoprotein catabolism. We expect to find that changes in the type or number of hepatic lipoprotein receptors will be reflected in changes in the receptor-mediated catabolism of lipoproteins. Cultured hepatocyutes will be used in the second procedure to determine the function of these receptors. The tyupe and number of receptors on cultured hepatocytes will be determined by competitive binding experiments and Scatchard analysis of the binding of different lipoproteins. The cells will also be pre-incubated with lipoproteins or other agents with the goal of inducing one receptor and suppressing the other so that the function of each can be studied. With incubation of various lipoproteins the receptor-mediated changes in the following functions will be measures: Cholesterol esterification, HMG Co-A reductase, cholesterol esterification and bile acid synthesis and secretion. These biochemical and physiological studies of the hepatic lipoprotein receptors will help us understand how cholesterol levels are regulated.